1. Field
This disclosure relates generally to memory devices, and more specifically, to magnetic memory devices
2. Related Art
Non-volatile memory devices are an important component in electronic systems. FLASH is the major non-volatile memory device in use today. Typical non-volatile memory devices use charges trapped in a floating oxide layer to store information. Disadvantages of FLASH memory include high voltage requirements, slow program and erase times, low endurance, and limitations of scalability.
To overcome these shortcomings, the semiconductor industry is evaluating magnetic memory devices, such as magnetoresistive random access memory (MRAM). A memory state in MRAM is maintained by the direction of the magnetic moment, not by charges stored in a layer. MRAM includes a magnetic tunnel junction (MTJ) structure. The MTJ has a fixed magnetic layer separated from a free magnetic layer by a dielectric tunnel barrier. In conventional MRAM, data storage is accomplished by applying magnetic fields and causing the free layer to be magnetized either parallel or antiparallel to the fixed layer magnetization, corresponding to the two possible memory states. Recalling data is accomplished by sensing the resistance of a current tunneling between the free and fixed magnetic layers. If the magnetic moment of the free layer is parallel to the fixed layer moment, then the MRAM device has a low resistance. If, instead, the magnetic moment of the free layer is antiparallel to the fixed layer moment, the MRAM device has a high resistance. The magnetic fields for writing are created by passing currents through conductive lines external to the MTJ structure.
Different types of MRAM exist. One type is a toggle MRAM. In toggle MRAM, bits are programmed using magnetic fields that are generated by passing currents down conducting lines that are in close proximity to the bits. The same pulse sequence is used to switch from one state (e.g., a “1” or a “0”) to another state (e.g., a “0” or a “1.”) While toggle MRAM has some advantages, it does not achieve a small write current, especially for small cell sizes. Another type of MRAM, spin torque MRAM (STMRAM In ST-MRAM, the bits are programmed by allowing a spin-polarized electron current to impinge upon a magnetic free layer. The change of angular momentum associated with the spin-polarized current generates a torque on the free layer that can change its magnetization direction, for large enough currents. While ST-MRAM uses substantially less write current than a toggle MRAM, particularly at small cell sizes, the write current for STMRAM is still undesirably large and for some cells, requires a programming voltage that exceeds the breakdown voltage of the tunnel barrier (Vbd). In addition, a large write current requires a large pass transistor for selectively programming the bit in an array, thereby undesirably limiting the overall memory density. Therefore, a need exists to further decrease the programming currents for spin transfer effect devices.